Problem: On the first 4 tests of her math class, Jessica got an average score of 83. What does she need on the next test to have an overall average of 85?
Explanation: Let her score on the next test be $x$ The sum of all of her scores is then $4 \cdot 83 + x$ The same sum must also be equal to $5 \cdot 85$ Solve: $x = 5 \cdot 85 - 4 \cdot 83 = 93$.